Fix warnings about undeclared corejs version

[mtlynch]

The npm run build command currently generates this warning about core-js:

WARNING: We noticed you're using the `useBuiltIns` option without declaring a core-js version. Currently, we assume version 2.x when no version is passed. Since this default version will likely change in future versions of Babel, we recommend explicitly setting the core-js version you are using via the `corejs` option.

You should also be sure that the version you pass to the `corejs` option matches the version specified in your `package.json`'s `dependencies` section. If it doesn't, you need to run one of the following commands:

  npm install --save core-js@2    npm install --save core-js@3
  yarn add core-js@2              yarn add core-js@3

I tried installing core-js to my package.json but it's already installed.

I found this solution which says to add settings to my .babelrc, and I tried adding them to my babel.config.js but I just ended up with this error:

Module build failed (from ./node_modules/babel-loader/lib/index.js): ReferenceError: [BABEL] /samba/whatgotdone/frontend/src/main.js: Unknown option:  .useBuiltIns. Check out https://babeljs.io/docs/en/babel-core/#options for more information about options.

[rasfa98]

@mtlynch I can take a look at this 🙂

[mtlynch]

This issue is still open if anyone from #hacktoberfest wants to take a look.

[smartrack]

@mtlynch I tried npm run build in my local. I don't see this warning message

well I see different warning message. Can we close this issue?

[mtlynch]

Yeah, looks like it's no longer happening. Thanks!